Method for recycling denitration catalyst

ABSTRACT

There is provided a method for recycling a catalyst that exhibits a high denitration efficiency at a relatively low temperature and does not cause oxidation of SO2 in a selective catalytic reduction reaction that uses ammonia as a reducing agent. A method for recycling a denitration catalyst includes a step of removing a used denitration catalyst from a denitration device and then coating the used denitration catalyst with a catalyst component. The catalyst component contains 43 wt % or more of vanadium pentoxide and has a BET specific surface area of 30 m2/g or more, and the denitration catalyst after recycling is used for denitration at 200° C. or lower.

TECHNICAL FIELD

The present invention relates to a method for recycling a denitration catalyst. More specifically, the present invention relates to a method for recycling a denitration catalyst used when exhaust gas generated through combustion of fuel is cleaned up.

BACKGROUND ART

One of pollutants emitted to the air through fuel combustion is nitrogen oxide (NO, NO₂, NO₃, N₂O, N₂O₃, N₂O₄, or N₂O₅). Nitrogen oxide causes, for example, acid rain, ozone depletion, and photochemical smog and seriously affects the environment and the human body, and therefore the treatment for nitrogen oxide has been an important issue.

A known technique of removing the nitrogen oxide is a selective catalytic reduction reaction (NH₃-SCR) that uses ammonia (NH₃) as a reducing agent. As described in Patent Document 1, a catalyst in which vanadium oxide is supported on titanium oxide serving as a carrier is widely used as a catalyst for the selective catalytic reduction reaction. Titanium oxide is the best carrier because titanium oxide has a low activity against sulfur oxide and has high stability.

Patent Document 1: Japanese Unexamined Patent Application, Publication No. 2004-275852

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

On the other hand, vanadium oxide plays a key role in the NH₃-SCR, but vanadium oxide cannot be supported in an amount of about 1 wt % or more because vanadium oxide oxidizes SO₂ into SO₃. Therefore, vanadium oxide is typically used in an amount of 1 wt % or less relative to its carrier. Furthermore, in the current NH₃-SCR, a catalyst in which vanadium oxide (and tungsten oxide in some cases) is supported on a titanium oxide carrier hardly reacts at low temperature and thus needs to be used at a high temperature of 350° C. to 400° C.

In order to increase the degree of freedom in the design of apparatuses and facilities with which the NH₃-SCR is performed and increase the efficiency, the development of a catalyst having a high nitrogen oxide reduction activity even at low temperature has been demanded. With this development, a method for recycling such a catalyst has also been demanded.

In view of the foregoing, it is an object of the present invention to provide a catalyst that exhibits a high denitration efficiency at low temperature and does not cause oxidation of SO₂ in a selective catalytic reduction reaction that uses ammonia as a reducing agent.

Means for Solving the Problems

The present invention relates to a method for recycling a denitration catalyst, the method including a step of removing a used denitration catalyst from a denitration device and then coating the used denitration catalyst with a catalyst component, wherein the catalyst component contains 43 wt % or more of vanadium pentoxide and has a BET specific surface area of 30 m²/g or more, and the denitration catalyst after recycling is used for denitration at 200° C. or lower.

The used denitration catalyst is preferably coated with the catalyst component by impregnating the used denitration catalyst with a chelate compound solution in which vanadium pentoxide is dissolved and then performing firing.

The used denitration catalyst is preferably coated with the catalyst component by spraying a chelate compound solution in which vanadium pentoxide is dissolved onto the used denitration catalyst and then performing firing.

In the catalyst component, an amount of NH₃ desorbed by NH₃-TPD (TPD: temperature programmed desorption) is preferably 10.0 μmol/g or more.

Effects of the Invention

The denitration catalyst that has been recycled by the recycling method according to the present invention exhibits a high denitration efficiency particularly at 200° C. or lower, which allows detoxification of NO into N₂. The selective catalytic reduction reaction that uses the denitration catalyst which has been recycled by the recycling method according to the present invention can be performed at 200° C. or lower, which does not cause oxidation of SO₂.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the powder X-ray diffraction results of vanadium pentoxide catalysts produced in Example 1, Reference Examples 1 and 2, and Comparative Example 1.

FIG. 2 illustrates the powder X-ray diffraction results of vanadium pentoxide catalysts produced in Examples 1 and 2, Reference Examples 3 to 6, and Comparative Examples 2 and 3.

FIG. 3 illustrates the NH₃-SCR activity of vanadium pentoxide catalysts produced in Example 1, Reference Examples 1 and 2, and Comparative Examples 1 and 4.

FIG. 4 illustrates the relationship between the reaction temperature and the N₂ selectivity in a selective catalytic reduction reaction that uses vanadium pentoxide catalysts produced in Reference Example 1 and Comparative Example 1.

FIG. 5 illustrates the space velocity dependency in the case where a vanadium pentoxide catalyst produced in Reference Example 1 is used in an NH₃-SCR reaction.

FIG. 6 illustrates a change in the NO conversion ratio over time in the case where a vanadium pentoxide catalyst produced in Reference Example 1 is used in a selective catalytic reduction reaction in coexistence with water.

FIG. 7 illustrates changes in the NH₃, NO, and SO₂ concentrations over time in the case where a vanadium pentoxide catalyst produced in Reference Example 1 is used in a selective catalytic reduction reaction in coexistence with S.

FIG. 8 illustrates the relationship between the amount of vanadium pentoxide supported and the NO conversion ratio of a vanadium pentoxide catalyst produced in each of Examples at each reaction temperature.

FIG. 9 illustrates the relationship between the BET specific surface area and the NO conversion ratio of a vanadium pentoxide catalyst produced in each of Examples, Reference Examples, and Comparative Examples.

FIG. 10 illustrates the powder X-ray diffraction results of vanadium pentoxide catalysts produced in Examples 4 to 6 and Reference Examples 7 and 8.

FIG. 11 illustrates the NH₃-SCR activity of vanadium pentoxide catalysts produced in Examples 4 to 6 and Reference Examples 7 and 8.

FIG. 12 illustrates the relationship between the specific surface area and the NO conversion ratio of vanadium pentoxide catalysts produced in Examples 4 to 6, Reference Examples 1, 2, and 7, and Comparative Example 1.

FIG. 13 illustrates the relationship between the BET specific surface area and the amount of NH₃ desorbed of vanadium pentoxide catalysts produced in Examples 4 and 5, Reference Examples 1 and 2, and Comparative Example 1.

FIG. 14 illustrates the relationship between the amount of NH₃ desorbed and the NO conversion ratio of vanadium pentoxide catalysts produced in Examples 4 and 5, Reference Examples 1 and 2, and Comparative Example 1.

PREFERRED MODE FOR CARRYING OUT THE INVENTION

Hereafter, embodiments of the present invention will be described.

A method for recycling a denitration catalyst according to the present invention includes a step of removing a used denitration catalyst from a denitration device and then coating the used denitration catalyst with a catalyst component. The catalyst component contains 43 wt % or more of vanadium pentoxide and has a BET specific surface area of 30 m²/g or more, and the denitration catalyst after recycling is used for denitration at 200° C. or lower. Such a denitration catalyst can exhibit a high denitration effect even in a low-temperature environment compared with known denitration catalysts such as a vanadium/titanium catalyst.

Specifically, when a denitration catalyst containing 3.3 wt % or more of vanadium oxide in terms of vanadium pentoxide is used in a selective catalytic reduction reaction (NH₃-SCR) that uses ammonia as a reducing agent, the NO conversion ratio is approximately 35% or more at a reaction temperature of 120° C. and approximately 60% or more at a reaction temperature of 150° C. Even at a reaction temperature of 100° C., the NO conversion ratio exceeds 20%. In contrast, if the denitration catalyst contains only less than 3.3 wt % of vanadium oxide in terms of vanadium pentoxide, the NO conversion ratio is less than 20% at a reaction temperature of 120° C. and even at a reaction temperature of 150° C.

As described above, the catalyst component used for coating in the method for recycling a denitration catalyst according to the present invention contains 43 wt % or more of vanadium oxide in terms of vanadium pentoxide, and may also contain titanium oxide as another component in addition to the vanadium oxide. Furthermore, a noble metal, a base metal, and a main group metal may be contained. Preferably, for example, tungsten oxide, chromium oxide, and molybdenum oxide can also be contained.

It has been described that the catalyst component used for coating in the method for recycling a denitration catalyst according to the present invention preferably contains 43 wt % or more of vanadium oxide in terms of vanadium pentoxide. Preferably, the catalyst component may contain 80 wt % or more of vanadium oxide in terms of vanadium pentoxide. More preferably, the content of vanadium oxide in the catalyst component may be 100%.

The above-described vanadium oxide includes vanadium(II) oxide (VO), vanadium(III) trioxide (V₂O₃), vanadium(IV) dioxide (V₂O₄), and vanadium(V) pentoxide (V₂O₅), and the V element in vanadium pentoxide (V₂O₅) may have a pentavalent, tetravalent, trivalent, or divalent form in the denitration reaction.

Regarding the BET specific surface area of the catalyst component, for example, in the NH₃-SCR that is performed at a reaction temperature of 120° C. using a denitration catalyst containing vanadium pentoxide and having a BET specific surface area of 13.5 m²g⁻¹, the NO conversion ratio exceeds 20%. Even in the NH₃-SCR that is performed at a reaction temperature of 120° C. using a denitration catalyst containing vanadium pentoxide and having a BET specific surface area of 16.6 m²g⁻¹, the NO conversion ratio exceeds 20%. In contrast, in the NH₃-SCR that is performed at a reaction temperature of 120° C. using, for example, a denitration catalyst having a BET specific surface area of 4.68 m²/g, which is a denitration catalyst having a BET specific surface area of less than 10 m²/g, the NO conversion ratio falls below 20%.

The BET specific surface area of the catalyst component is 10 m²/g or more and may be preferably 15 m²/g or more. More preferably, the BET specific surface area of the catalyst component may be 30 m²/g. More preferably, the BET specific surface area of the catalyst component may be 40 m²/g or more. More preferably, the BET specific surface area of the catalyst component may be 50 m²/g or more. More preferably, the BET specific surface area of the catalyst component may be 60 m²/g or more.

The BET specific surface area of the catalyst component is preferably measured in conformity with the conditions specified in JIS Z 8830:2013. Specifically, the BET specific surface area can be measured by a method described in Examples below.

The denitration catalyst recycled by the recycling method of the present invention is used for denitration at 200° C. or lower. Preferably, the denitration catalyst is used for denitration at 160° C. or higher and 200° C. or lower. Thus, oxidation of SO₂ into SO₃ does not occur during the NH₃-SCR reaction.

Regarding the amount of NH₃ desorbed by NH₃-TPD (TPD: temperature programmed desorption), when the amount of NH₃ desorbed exceeds 10.0 μmol/g, the NO conversion ratio of the denitration catalyst in the NH₃-SCR at a reaction temperature of 120° C. is 20% or more. In contrast, when the amount of NH₃ desorbed falls below 10.0 μmol/g, the NO conversion ratio of the denitration catalyst in the NH₃-SCR at a reaction temperature of 120° C. falls below 20%.

In the catalyst component used for coating in the recycling method of the present invention, the amount of NH₃ desorbed by NH₃-TPD (TPD: temperature programmed desorption) is 10.0 μmol/g or more. Preferably, the amount of NH₃ desorbed by NH₃-TPD may be 20.0 μmol/g or more. More preferably, the amount of NH₃ desorbed by NH₃-TPD may be 50.0 μmol/g or more. More preferably, the amount of NH₃ desorbed by NH₃-TPD may be 70.0 μmol/g or more.

The catalyst component of the denitration catalyst containing 43 wt % or more of vanadium pentoxide and having a BET specific surface area of 10 m²/g or more can be produced by any of a thermal decomposition process, a sol-gel process, and an impregnation process. Hereafter, a method for producing the denitration catalyst containing 3.3 wt % or more of vanadium pentoxide and having a specific surface area of 10 m²/g or more by a thermal decomposition process, a sol-gel process, or an impregnation process will be described.

The thermal decomposition process includes a step of thermally decomposing a vanadate. Examples of the vanadate that may be used include ammonium vanadate, magnesium vanadate, strontium vanadate, barium vanadate, zinc vanadate, tin vanadate, and lithium vanadate.

In the thermal decomposition process, the vanadate is preferably thermally decomposed at 300° C. to 400° C.

The sol-gel process includes a step of dissolving a vanadate in a chelate compound, performing drying, and performing firing. Examples of the chelate compound that may be used include compounds having a plurality of carboxy groups, such as oxalic acid and citric acid; compounds having a plurality of amino groups, such as acetylacetonate and ethylenediamine; and compounds having a plurality of hydroxy groups, such as ethylene glycol.

The sol-gel process preferably includes a step of dissolving a vanadate in a chelate compound such that the molar ratio of vanadium and the chelate compound is, for example, 1:1 to 1:5, though this is dependent on the chelate compound. Preferably, the molar ratio of the vanadate and the chelate compound may be 1:2 to 1:4.

The impregnation process includes a step of dissolving a vanadate in a chelate compound, adding a carrier, performing drying, and then performing firing. Examples of the carrier that may be used include titanium oxide, aluminum oxide, and silica. As above, examples of the chelate compound that may be used include compounds having a plurality of carboxy groups, such as oxalic acid and citric acid; compounds having a plurality of amino groups, such as acetylacetonate and ethylenediamine; and compounds having a plurality of hydroxy groups, such as ethylene glycol.

In the impregnation process, xwt % V₂O₅/TiO₂ (x≥43) may be produced as the denitration component of the denitration catalyst according to an embodiment of the present invention by, for example, dissolving ammonium vanadate in an oxalic acid solution, adding titanium oxide (TiO₂) serving as a carrier, performing drying, and then performing firing.

The thus-produced denitration component of the denitration catalyst normally contains 3.3 wt % or more of vanadium pentoxide and has a specific surface area of 10 m²/g or more.

In the case where the surface of the denitration catalyst is unusably deteriorated by various substances contained in the fuel after the denitration catalyst is set in a denitration device, the used denitration catalyst can be recycled by removing the used denitration catalyst from a catalyst device and coating the surface of the used denitration catalyst with the above catalyst component.

Specifically, a used denitration catalyst removed from a denitration device is immersed in a chelate compound solution in which vanadium pentoxide is dissolved, dried, and then fired. Thus, the used denitration catalyst can be recycled by being coated with vanadium pentoxide. Alternatively, a used denitration catalyst removed from a denitration device is subjected to spraying with a chelate compound solution in which vanadium pentoxide is dissolved, dried, and then fired. Thus, the used denitration catalyst can be recycled by being coated with vanadium pentoxide. As above, examples of the chelate compound that may be used include compounds having a plurality of carboxy groups, such as oxalic acid and citric acid; compounds having a plurality of amino groups, such as acetylacetonate and ethylenediamine; and compounds having a plurality of hydroxy groups, such as ethylene glycol.

The molar ratio of the vanadium pentoxide and the chelate compound is preferably 1:1 to 1:5. Preferably, the molar ratio of the vanadium pentoxide and the chelate compound may be 1:2 to 1:4. The firing temperature is preferably, for example, about 300° C.

The denitration catalyst according to the above embodiment produces the following effects.

(1) As described above, the method for recycling a denitration catalyst according to the above embodiment includes a step of removing a used denitration catalyst from a denitration device and then coating the used denitration catalyst with a catalyst component. The catalyst component contains 43 wt % or more of vanadium pentoxide and has a BET specific surface area of 10 m²/g or more. The denitration catalyst after recycling is used for denitration at 200° C. or lower. By using this denitration catalyst recycled by the recycling method according to the above embodiment, a high denitration effect can be produced even in a selective catalytic reduction reaction at 200° C. or lower. Furthermore, in the selective catalytic reduction reaction that uses the denitration catalyst recycled by the recycling method according to the above embodiment, a high denitration effect is produced without oxidizing SO₂.

(2) As described above, the used denitration catalyst is preferably coated with the catalyst component by impregnating the used denitration catalyst with a chelate compound solution in which vanadium pentoxide is dissolved and then performing firing. This increases the specific surface area of the denitration catalyst recycled by the recycling method according to the above embodiment, which improves the denitration effect in the selective catalytic reduction reaction that uses the denitration catalyst according to the above embodiment.

(3) As described above, the used denitration catalyst is preferably coated with the catalyst component by spraying a chelate compound solution in which vanadium pentoxide is dissolved onto the used denitration catalyst and then performing firing. This increases the specific surface area of the denitration catalyst recycled by the recycling method according to the above embodiment, which improves the denitration effect in the selective catalytic reduction reaction that uses the denitration catalyst according to the above embodiment.

(4) As described above, in the catalyst component, the amount of NH₃ desorbed by NH₃-TPD (TPD: temperature programmed desorption) is preferably 10.0 μmol/g or more. Thus, when the denitration catalyst recycled by the recycling method according to the above embodiment is used in the NH₃-SCR at a reaction temperature of 120° C., the NO conversion ratio exceeds 20%.

The present invention is not limited to the above embodiment, and any of modifications, improvements, and the like are included in the present invention as long as the object of the present invention is achieved.

EXAMPLES

Hereafter, Examples of the catalyst component of the present invention will be specifically described together with Reference Examples and Comparative Examples. The present invention is not limited by Examples.

1. Relationship Between Vanadium Oxide Content and Specific Surface Area and NH₃-SCR Activity 1.1 Examples and Comparative Examples Reference Example 1

Ammonium vanadate (NH₄VO₃) was thermally decomposed in the air at 300° C. for 4 hours to obtain vanadium pentoxide (V₂O₅). The obtained vanadium pentoxide was used as a denitration catalyst in Reference Example 1. The sample name of the denitration catalyst in Reference Example 1 was “V₂O₅ _(_)300”.

Reference Example 2

Ammonium vanadate was thermally decomposed in the air at 400° C. for 4 hours to obtain vanadium pentoxide. The obtained vanadium pentoxide was used as a denitration catalyst in Reference Example 2. The sample name of the denitration catalyst in Reference Example 2 was “V₂O₅ _(_)400”.

Comparative Example 1

Ammonium vanadate was thermally decomposed in the air at 500° C. for 4 hours to obtain vanadium pentoxide. The obtained vanadium pentoxide was used as a denitration catalyst in Comparative Example 1. The sample name of the denitration catalyst in Comparative Example 1 was “V₂O₅ _(_)500”.

Example 1

Ammonium vanadate was dissolved in an oxalic acid solution (molar ratio of vanadium:oxalic acid=1:3). After ammonium vanadate was completely dissolved, water in the solution was evaporated on a hot stirrer, and drying was performed in a dryer at 120° C. for one night. Then, the dried powder was fired in the air at 300° C. for 4 hours. The vanadium pentoxide after firing was used as a denitration catalyst in Example 1. The sample name of the denitration catalyst in Example 1 obtained by this sol-gel process was “V₂O₅ _(_)SG_300”. Denitration catalysts obtained at different molar ratios of vanadium and oxalic acid when ammonium vanadate is dissolved in an oxalic acid solution will be described later.

Comparative Example 2

Ammonium vanadate was added to an oxalic acid solution and stirred for 10 minutes, and titanium oxide serving as a carrier was gradually added. Then, water in the solution was evaporated on a hot stirrer and drying was performed in a dryer at 120° C. for one night. Subsequently, the dried powder was fired in the air at 300° C. for 4 hours. As a result, the denitration catalyst after firing that contained 0.3 wt % of vanadium pentoxide was used as a denitration catalyst in Comparative Example 2. The sample name of the denitration catalyst in Comparative Example 2 was “0.3 wt % V₂O₅/TiO₂”.

Comparative Example 3

The denitration catalyst after firing that was obtained by the same method as in Comparative Example 2 and contained 0.9 wt % of vanadium pentoxide was used as a denitration catalyst in Comparative Example 3. The sample name of the denitration catalyst in Comparative Example 3 was “0.9 wt % V₂O₅/TiO₂”.

Reference Example 3

The denitration catalyst after firing that was obtained by the same method as in Comparative Example 2 and contained 3.3 wt % of vanadium pentoxide was used as a denitration catalyst in Reference Example 3. The sample name of the denitration catalyst in Reference Example 3 was “3.3 wt % V₂O₅/TiO₂”.

Reference Example 4

The denitration catalyst after firing that was obtained by the same method as in Comparative Example 2 and contained 9 wt % of vanadium pentoxide was used as a denitration catalyst in Reference Example 4. The sample name of the denitration catalyst in Reference Example 4 was “9 wt % V₂O₅/TiO₂”.

Reference Example 5

The denitration catalyst after firing that was obtained by the same method as in Comparative Example 2 and contained 20 wt % of vanadium pentoxide was used as a denitration catalyst in Reference Example 5. The sample name of the denitration catalyst in Reference Example 5 was “20 wt % V₂O₅/TiO₂”.

Reference Example 6

The denitration catalyst after firing that was obtained by the same method as in Comparative Example 2 and contained 33 wt % of vanadium pentoxide was used as a denitration catalyst in Reference Example 6. The sample name of the denitration catalyst in Reference Example 6 was “33 wt % V₂O₅/TiO₂”.

Example 2

The denitration catalyst after firing that was obtained by the same method as in Comparative Example 2 and contained 43 wt % of vanadium pentoxide was used as a denitration catalyst in Example 2. The sample name of the denitration catalyst in Example 2 was “43 wt % V₂O₅/TiO₂”.

Example 3

The denitration catalyst after firing that was obtained by the same method as in Comparative Example 2 and contained 80 wt % of vanadium pentoxide was used as a denitration catalyst in Example 3. The sample name of the denitration catalyst in Example 3 was “80 wt % V₂O₅/TiO₂”.

Comparative Example 4

An existing catalyst was used in Comparative Example 4. The existing catalyst is a catalyst in which, for example, tungsten oxide (WO₃) (content: 10.72 wt %) and silica (SiO₂) (content: 6.25 wt %) are supported on titanium oxide (TiO₂) (content: 79.67 wt %) and which contains about 0.5% of vanadium.

1.2 Evaluation 1.2.1 Powder X-Ray Diffraction

(Diffraction Method)

Powder X-ray diffraction analysis was performed with a Rigaku smart lab using Cu-Ka.

(Diffraction Result)

FIG. 1 illustrates powder XRD patterns of Example 1 (V₂O₅ _(_)SG_300), Reference Example 1 (V₂O₅ _(_)300), Reference Example 2 (V₂O₅ _(_)400), and Comparative Example 1 (V₂O₅ _(_)500). FIG. 2 illustrates powder XRD patterns of Example 1 (V₂O₅ _(_)SG_300) and Example 2, Reference Examples 3 to 6, and Comparative Examples 2 and 3 (xwt % V₂O₅/TiO₂). In the powder XRD patterns of Example 1 (V₂O₅ _(_)SG_300), Reference Example 1 (V₂O₅ _(_)300), Reference Example 2 (V₂O₅ _(_)400), and Comparative Example 1 (V₂O₅ _(_)500), only peaks for V₂O₅ were observed regardless of the thermal decomposition temperature and the production method. In the powder XRD patterns of Example 2, Reference Examples 3 to 6, and Comparative Examples 2 and 3 (xwt % V₂O₅/TiO₂), peaks for V₂O₅ were not observed at 9 wt % or less and thus V₂O₅ is believed to be highly dispersed in TiO₂. When the amount of V₂O₅ supported was increased to 20 wt %, peaks for V₂O₅ were observed at 22.2° and 27.4°, and the V₂O₅ peak intensity increased as the amount of V₂O₅ supported was increased. On the other hand, the TiO₂ peak intensity tended to decrease.

1.2.2 Measurement of BET Specific Surface Area

(Measurement Method)

The BET specific surface area was measured with a MicrotracBEL BELSORP-max. Pretreatment was performed in an Ar atmosphere at 200° C. for 2 hours, and then measurement was performed at 196° C.

(Measurement Result)

TABLE 1 BET specific surface area of vanadium pentoxide catalyst BET specific Sample surface area/m²g⁻¹ Reference Example1 (V₂O₅ _(—) 300) 16.6 Reference Example2 (V₂O₅ _(—) 400) 13.5 Comparative Example1 (V₂O₅ _(—) 500) 4.68 Example1 (V₂O₅ _(—) SG_300) 62.9 Comparative Example2 (0.3 wt % V₂O₅/TiO₂) 62.8 Comparative Example3 (0.9 wt % V₂O₅/TiO₂) 59 Reference Example3 (3.3 wt % V₂O₅/TiO₂) 55.4 Reference Example4 (9 wt % V₂O₅/TiO₂) 54.6 Reference Example5 (20 wt % V₂O₅/TiO₂) 48.3 Reference Example6 (33 wt % V₂O₅/TiO₂) 41.2 Example2 (43 wt % V₂O₅/TiO₂) 49.4 Example3 (80 wt % V₂O₅/TiO₂) 34 Comparative Example4 (Existing catalyst) 61.8

Table 1 shows BET specific surface areas of Reference Example 1 (V₂O₅ _(_)300), Reference Example 2 (V₂O₅ _(_)400), Comparative Example 1 (V₂O₅ _(_)500), Example 1 (V₂O₅ _(_)SG_300), Comparative Examples 2 and 3, Reference Examples 3 to 6, and Examples 2 and 3 (xwt % V₂O₅/TiO₂ catalyst), and Comparative Example 4 (existing catalyst). In the vanadium pentoxide catalysts obtained by thermally decomposing ammonium vanadate, the BET specific surface area decreased with increasing the thermal decomposition temperature. That is, the vanadium pentoxide in Reference Example 1 (V₂O₅ _(_)300) in which the thermal decomposition was performed at 300° C. had a maximum BET specific surface area of 16.6 m²g⁻¹. The vanadium pentoxide obtained at 300° C. through a sol-gel process had a larger BET specific surface area of 62.9 m²g⁻¹. In Reference Examples 3 to 6, Examples 2 and 3, and Comparative Examples 2 and 3 (xwt % V₂O₅/TiO₂), as the amount of vanadium pentoxide supported was increased, pores in TiO₂ were filled and the BET specific surface area decreased.

1.2.3 Measurement of Catalytic Activity

(Measurement Method)

An NH₃-SCR reaction was performed using a fixed-bed flow reactor under the conditions listed in Table 2 below. Among gases that had passed through the catalytic layer, NO, NH₃, NO₂, and N₂O were analyzed with a Jasco FT-IR-4700.

TABLE 2 NH₃-SCR measurement conditions Amount of catalyst 0.375 mg Gas flow rate 250 mLmin⁻¹ (NO: 250 ppm, NH₃: 250 ppm, O₂: 4 vol %) (2000 ppm NO/Ar 31.3 mL min⁻¹) (2000 ppm NH₃/Ar 31.3 mL min⁻¹) (O₂ 14 mL min⁻¹) (Ar 177.4 mL min⁻¹) Space velocity 40,000 ⁻¹ mLh⁻¹g_(cat)

Furthermore, the NO conversion ratio and the N₂ selectivity were calculated from formulae below. Herein, NO_(in) represents a NO concentration at an inlet of a reaction tube, NO_(out) represents a NO concentration at an outlet of the reaction tube, N_(2out) represents a N₂ concentration at the outlet of the reaction tube, NH_(3in) represents an NH₃ concentration at the inlet of the reaction tube, and NH_(3out) represents an NH₃ concentration at the outlet of the reaction tube.

$\begin{matrix} {{{NO}\mspace{14mu} {CONVERSION}\mspace{14mu} {RATIO}} = {\frac{{NO}_{in} - {NO}_{out}}{{NO}_{in}} \times 100}} & \left\lbrack {{Formula}.\mspace{14mu} 1} \right\rbrack \\ {{{N_{2}\mspace{14mu} {{SELECTIVITY}(\%)}} = {\frac{2*N_{2{out}}}{\left( {{NO}_{in} + {NH}_{3{in}}} \right) - \left( {{NO}_{out} + {NH}_{3{out}}} \right)} \times 100}}\left( {{2*N_{2{out}}} = {\left( {{NO}_{in} + {NH}_{3{in}}} \right) - \left( {{NO}_{out} + {NH}_{3{out}} + {NO}_{2{out}} + {2*N_{2}O_{out}}} \right)}} \right)} & \left\lbrack {{Formula}.\mspace{14mu} 2} \right\rbrack \end{matrix}$

(Measurement Result)

FIG. 3 illustrates the NH₃-SCR activity of the vanadium pentoxide catalysts. In the case of the catalysts obtained by thermally decomposing ammonium vanadate, the NO conversion ratio increased as the thermal decomposition temperature was decreased. The highest activity was exhibited in Reference Example 1 (V₂O₅ _(_)300° C.) in which the catalyst was obtained at a thermal decomposition temperature of 300° C. At a reaction temperature of 200° C., a NO conversion ratio of 80% or more was achieved when any of the catalysts in Reference Example 1 (V₂O₅ _(_)300° C.), Reference Example 2 (V₂O₅ 400° C.), and Example 1 (V₂O₅ _(_)SG_300° C.) was used. Furthermore, the NO conversion ratio was higher in any of Examples than in Comparative Example 1 and Comparative Example 4.

The specific surface area of the vanadium pentoxide increases as the thermal decomposition temperature is decreased. Therefore, it is believed that the low-temperature NH₃-SCR activity that uses a bulk vanadium pentoxide catalyst is attributable to the BET specific surface area. Hence, as described above, the vanadium pentoxide was produced through a sol-gel process that uses oxalic acid in order to increase the BET specific surface area in Example 1. The BET specific surface area of the vanadium pentoxide produced through this process is 62.9 m²g⁻¹ as shown in Table 1, which is about four times larger than the BET specific surface areas of the vanadium pentoxides produced through a thermal decomposition process. The NO conversion ratio in Example 1 (V₂O₅ _(_)SG_300° C.) was increased by 80% to 200% at 100° C. to 150° C. compared with the vanadium pentoxides produced through a thermal decomposition process.

The N₂ selectivity was almost 100% at any temperature. FIG. 4 illustrates, as examples, the N₂ selectivities in Reference Example 1 (V₂O₅ _(_)300° C.) and Comparative Example 1 (V₂O₅ _(_)500° C.).

(Space Velocity Dependency)

In the case where the catalyst in Reference Example 1 (V₂O₅ _(_)300° C.) was used, the space velocity (for gas treatment) dependency was measured by performing the selective catalytic reduction reaction under the conditions listed in Table 3 below. FIG. 5 illustrates the measurement results. FIG. 5(a) illustrates the NO conversion ratio at a reaction temperature of 120° C. FIG. 5(b) illustrates the NO conversion ratio at a reaction temperature of 100° C. The 80% NO detoxification was about 15 Lh⁻¹g_(cat) ⁻¹ at 120° C. and about 11 Lh⁻¹g_(cat) ⁻¹ at 100° C. In an experiment in which the space velocity was changed, the N₂ selectivity was almost 100%.

TABLE 3 NH₃-SCR measurement conditions Reaction temperature 120 or 100° C. Amount of catalyst 0.375 g, 0.750 g, 1.5 g Total gas flow rate 250 mLmin⁻¹ (NO: 250 ppm, NH₃: 250 ppm, O₂: 4 vol %, Ar balance) Space velocity 10-40 Lh⁻¹g_(cat) ⁻¹ Gas flow time 0.5 h

(Reaction in Coexistence with Water)

An experiment of the NH₃-SCR reaction was performed using the catalyst in Reference Example 1 (V₂O₅ _(_)300° C.) under the conditions listed in Table 4 below at a reaction temperature of 150° C. at a space velocity of 20 Lh⁻¹g_(cat) ⁻¹. FIG. 6 illustrates a change in the NO conversion ratio over time in the experiment. As a result of addition of 2.3% H₂O 1.5 hours after the start of the reaction, the NO conversion ratio decreased from 64% to 50%. The addition of H₂O did not change the N₂ selectivity. The N₂ selectivity was 100%. As a result of stop of the addition of water 3.5 hours after the start of the reaction, the NO conversion ratio increased to 67%.

TABLE 4 NH₃-SCR measurement conditions Reaction temperature 150° C. Amount of catalyst 0.375 g Total gas flow rate 250 mLmin⁻¹ (NO: 250 ppm, NH₃: 250 ppm, O₂: 4 vol %, Ar balance) Space velocity 20 Lh⁻¹g_(cat) ⁻¹

(Reaction in Coexistence with S)

Under the same conditions as those of the experiment of the reaction in coexistence with water, 100 ppm SO₂ was caused to flow through a reaction gas. FIG. 7 illustrates the experimental results. No change occurred to the catalytic activity of NO. After the completion of the temperature increase to 150° C., the SO₂ concentration did not decrease though H₂O and O₂ were constantly present. Consequently, SO₂ did not react. Accordingly, the denitration catalysts in Examples were found to have S resistance.

(Relationship Between Amount of Vanadium Pentoxide Supported and NO Conversion Ratio)

FIG. 8 illustrates the relationship between the amount of vanadium pentoxide supported and the NO conversion ratio at each reaction temperature. FIG. 8(a) illustrates the relationship between the amount of vanadium pentoxide supported and the NO conversion ratio at a reaction temperature of 120° C. Similarly, FIG. 8(b) illustrates the relationship between the amount of vanadium pentoxide supported and the NO conversion ratio at a reaction temperature of 150° C., and FIG. 8(c) illustrates the relationship at a reaction temperature of 100° C. In each of the graphs, the catalyst in which the amount of vanadium pentoxide supported is 100 wt % is the denitration catalyst V₂O₅ _(_)SG_300 produced in Example 1. The points plotted using a square indicate a NO conversion ratio of the existing catalyst in Comparative Example 4. All the graphs showed that, on the whole, the NO conversion ratio increased as the amount of vanadium pentoxide supported was increased. Herein, all the graphs showed that the catalyst in which the amount of vanadium pentoxide supported was 3.3 wt % had a higher NO conversion ratio than the catalyst in which the amount of vanadium pentoxide supported was 9.0 wt %. Specifically, as illustrated in FIG. 8(a), in the NH₃-SCR reaction at a reaction temperature of 120° C., the NO conversion ratio reached 80% when the amount of vanadium pentoxide supported was increased to 80 wt %. As illustrated in FIG. 8(b), in the NH₃-SCR reaction at a reaction temperature of 150° C., the NO conversion ratio considerably increased when the amount of vanadium pentoxide supported was increased to 3.3 wt %. As illustrated in FIG. 8(c), in the selective catalytic reduction reaction at a reaction temperature of 100° C., the denitration catalyst in which the amount of vanadium pentoxide supported was 80 wt % had a considerably increased NO conversion ratio compared with the denitration catalysts in which the amounts of vanadium pentoxide supported were 43 wt % or less.

(Relationship Between BET Specific Surface Area and NO Conversion Ratio)

FIG. 9(a) illustrates the relationship between the BET specific surface area and the NO conversion ratio of the denitration catalysts in which vanadium pentoxide was supported on titanium oxide. In the denitration catalyst in which vanadium pentoxide was supported on titanium oxide, as the amount of vanadium pentoxide supported was increased, the BET specific surface area decreased, but the activity increased on the whole. FIG. 9(b) illustrates the relationship between the BET specific surface area and the NO conversion ratio of both the denitration catalysts in which vanadium pentoxide was supported on titanium oxide and the denitration catalysts in which vanadium pentoxide was not supported on titanium oxide. In the catalysts in which vanadium pentoxide was not supported on titanium oxide, the activity increased with increasing the BET specific surface area.

2. V₂O₅ Catalyst Produced Through Sol-Gel Process 2.1 Examples (Examples 4 to 6 and Reference Examples 7 and 8)

In “Example 1” of the above-described “1.1 Examples and Comparative Examples”, ammonium vanadate was dissolved in an oxalic acid solution such that the molar ratio of vanadium and oxalic acid was 1:3, then water was evaporated, drying was performed, and the resulting dried powder was fired. Thus, a denitration catalyst was produced. In the denitration catalysts of Reference Example 7, Examples 4 to 6, and Reference Example 8, the molar ratios of vanadium and oxalic acid were set to 1:1, 1:2, 1:3, 1:4, and 1:5, respectively. Specifically, as described above, ammonium vanadate was dissolved in an oxalic acid solution (molar ratio of vanadium:oxalic acid=1:1 to 1:5). After ammonium vanadate was completely dissolved, water in the solution was evaporated on a hot stirrer, and drying was performed in a dryer at 120° C. for one night. Then, the dried powder was fired in the air at 300° C. for 4 hours. The sample names were given as “V₂O₅ _(_)SG_1:1” (Reference Example 7), “V₂O₅ _(_)SG_1:2” (Example 4), “V₂O₅ _(_)SG_1:3” (Example 5), “V₂O₅ _(_)SG_1:4” (Example 6), and “V₂O₅ _(_)_SG_1:5” (Reference Example 8). Herein, the “V₂O₅ _(_)SG_300” in “Example 1” of “1.1 Examples and Comparative Examples” and “V₂O₅ _(_)SG_1:3” in Example 5 were substantially the same, but the sample name “V₂O₅ _(_)SG_1:3” in “Example 5” was used for the sake of convenience of description. To increase the BET specific surface area, a surfactant may be added to the oxalic acid solution. Examples of the surfactant include anionic surfactants such as hexadecyltrimethylammonium bromide (CTAB), sodium lauryl sulfate (SDS), and hexadecylamine; cationic surfactants; amphoteric surfactants; and nonionic surfactants.

2.2 Evaluation 2.2.1 Powder X-Ray Diffraction

(Diffraction Method)

In the same manner as in 1.2.1, powder X-ray diffraction analysis was performed with a Rigaku smart lab using Cu-Ka.

(Diffraction Result)

FIG. 10 illustrates powder XRD patterns of Reference Example 7, Examples 4 to 6, and Reference Examples 8 (V₂O₅ _(_)SG). In the vanadium pentoxides (Reference Examples 7, 7, and 10) produced using the solutions having vanadium:oxalic acid ratios of 1:1, 1:2, and 1:5, only peaks for orthorhombic V₂O₅ were detected. In the vanadium pentoxides (Examples 5 and 6) produced using the solutions having vanadium:oxalic acid ratios of 1:3 and 1:4, an unidentified peak was detected at 110 in addition to the peaks for orthorhombic V₂O₅. However, the peak has not been identified yet.

2.2.2 Measurement of BET Specific Surface Area

(Measurement Method)

In the same manner as in 1.2.3, the BET specific surface area was measured with a MicrotracBEL BELSORP-max. Pretreatment was performed in an Ar atmosphere at 200° C. for 2 hours, and then measurement was performed at 196° C.

(Measurement Result)

TABLE 5 BET specific surface area of vanadium pentoxide catalyst BET specific BET specific surface area before surface area after Catalyst reaction/m²g⁻¹ reaction/m²g⁻¹ Reference (V₂O₅ _(—) SG_1:1) 29.9 n.d. Example7 Example4 (V₂O₅ _(—) SG_1:2) 33.5 n.d. Example5 (V₂O₅ _(—) SG_1:3) 62.9 43.4 Example6 (V₂O₅ _(—) SG_1:4) 57.0 n.d. Reference (V₂O₅ _(—) SG_1:5) n.d. n.d. Example8

Table 5 shows BET specific surface areas of Reference Example 7 (V₂O₅ _(_)SG_1:1), Example 4 (V₂O₅ _(_)SG_1:2), Example 5 (V₂O₅ _(_)SG_1:3), Example 6 (V₂O₅ _(_)SG_1:4), and Reference Example 8 (V₂O₅ _(_)SG_1:5). As the ratio of the oxalic acid was increased, the specific surface area increased at vanadium:oxalic acid ratios of 1:1 to 1:3. When the ratio of the oxalic acid was further increased, the specific surface area decreased. The specific surface area in Example 5 (V₂O₅ _(_)SG_1:3) after the catalytic activity test described below considerably decreased to 43.4 m²g⁻¹ compared with the specific surface area before the catalytic activity test.

2.2.3 Measurement of Catalytic Activity

(Measurement Method)

By the same measurement method as in 1.2.4, the NH₃-SCR activity of each V₂O₅ _(_)SG catalyst was measured and the NO conversion ratio was calculated.

(Measurement Result)

FIG. 11 illustrates the NH₃-SCR activity of each V₂O₅ _(_)SG catalyst. FIG. 11(a) illustrates the NO conversion ratio plotted against reaction temperature in the NH₃-SCR reaction that uses each catalyst. FIG. 11(b) illustrates the relationship between the vanadium:oxalic acid ratio and the NO conversion ratio at a reaction temperature of 120° C. In the catalyst of Example 5 (V₂O₅ _(_)SG_1:3) having a vanadium:oxalic acid ratio of 1:3, the highest NO conversion ratio was achieved. When the oxalic acid was further added, the NO conversion ratio decreased. The NO conversion ratio in Example 6 (V₂O₅ _(_)SG_1:4) was lower than that in Example 4 (V₂O₅ _(_)SG_1:2) despite the fact that the specific surface area in Example 6 was larger than that in Example 4.

(Relationship Between Specific Surface Area and NO Conversion Ratio)

FIG. 12 illustrates the relationship between the BET specific surface area and the NO conversion ratio in Examples 4 to 6 and Reference Example 7 (V₂O₅ _(_)SG), Reference Example 1 (V₂O₅ _(_)300), Reference Example 2 (V₂O₅ _(_)400), and Comparative Example 1 (V₂O₅ _(_)500). The point plotted using a square indicates the relationship between the BET specific surface area and the NO conversion ratio after the selective catalytic reduction reaction in Example 5 (V₂O₅ _(_)_SG_1:3). As described above, it was shown that the highest NO conversion ratio was achieved in the catalyst of Example 5 (V₂O₅ _(_)SG_1:3) having a vanadium:oxalic acid ratio of 1:3.

2.2.4 Characterization by NH₃-TPD

(Measurement Method)

The amount of acid sites on the surface of the catalyst can be estimated by NH₃-TPD (TPD: temperature programmed desorption). In a BELCAT manufactured by MicrotracBEL Corp., 0.1 g of each of the catalysts in Reference Example 1 (V₂O₅ _(_)300), Reference Example 2 (V₂O₅ _(_)400), Comparative Example 1 (V₂O₅ _(_)500), Example 4 (V₂O₅ _(_)SG_1:2), and Example 5 (V₂O₅ _(_)SG_1:3) was pretreated at 300° C. for 1 hour while He (50 ml/min) was caused to flow. Then, the temperature was decreased to 100° C., and 5% ammonia/He (50 ml/min) was caused to flow for 30 minutes to adsorb ammonia. The flow gas was changed to He (50 ml/min) and this state was kept for 30 minutes for stabilization. Then, the temperature was increased at 10° C./min and ammonia, which has a mass number of 16, was monitored with a mass spectrometer.

(Measurement Result)

TABLE 6 Measured amount of NH₃ desorbed by NH₃-TPD Amount of NH₃ Catalyst desorbed/μmolg⁻¹ Reference Example1 (V₂O₅ _(—) 300) 22.9 Reference Example2 (V₂O₅ _(—) 400) 14.0 Comparative Example1 (V₂O₅ _(—) 500) 5.21 Example4 (V₂O₅ _(—) SG_1:2) 51.4 Example5 (V₂O₅ _(—) SG_1:3) 77.5

Table 6 shows the measurement results of the amount of NH₃ desorbed when the catalysts in Reference Example 1 (V₂O₅ _(_)300), Reference Example 2 (V₂O_400), Comparative Example 1 (V₂O_500), Example 4 (V₂O₅ _(_)SG_1:2), and Example 5 (V₂O₅ _(_)SG_1:3) were used.

FIG. 13 is a graph obtained by plotting the amount of NH₃ desorbed as a function of the BET specific surface area of each catalyst. The graph in FIG. 13 showed that the amount of NH₃ desorbed increased substantially in proportion to the BET specific surface area of V₂O₅. FIG. 14 is a graph obtained by plotting the NO conversion ratio as a function of the amount of NH₃ desorbed in each catalyst. The graph showed that the NO conversion ratio increased as the catalyst had a larger amount of NH₃ desorbed, that is, a larger amount of acid sites on the surface of the catalyst.

As described above, the denitration catalyst of the present invention that contains 3.3 wt % or more of vanadium oxide in terms of vanadium pentoxide and has a specific surface area of 10 m²/g or more exhibits a high denitration efficiency at a low temperature of 200° C. or lower in the selective catalytic reduction reaction that uses ammonia as a reducing agent. On the other hand, oxidation of SO₂ is not found. 

1. A method for recycling a denitration catalyst, comprising: a step of removing a used denitration catalyst from a denitration device and then coating the used denitration catalyst with a catalyst component, wherein the catalyst component contains 43 wt % or more of vanadium pentoxide and has a BET specific surface area of 30 m²/g or more, and the denitration catalyst after recycling is used for denitration at 200° C. or lower.
 2. The method for recycling a denitration catalyst according to claim 1, wherein the used denitration catalyst is coated with the catalyst component by impregnating the used denitration catalyst with a chelate compound solution in which vanadium pentoxide is dissolved and then performing firing.
 3. The method for recycling a denitration catalyst according to claim 1, wherein the used denitration catalyst is coated with the catalyst component by spraying a chelate compound solution in which vanadium pentoxide is dissolved onto the used denitration catalyst and then performing firing.
 4. The method for recycling a denitration catalyst according to claim 1, wherein in the catalyst component, an amount of NH₃ desorbed by NH₃-TPD (TPD: temperature programmed desorption) is 10.0 μmol/g or more. 